Shoulder replacement surgery and shoulder resurfacing surgery are used to treat severe arthritis or physical damage to the shoulder joint. In shoulder replacement surgeries, the head of the humerus is replaced with a metal ball. The glenoid fossa may also replaced with a plastic socket, covered with a new plastic surface, or ground smooth. When both components are replaced, this is referred to as a total shoulder replacement. When the head is replaced without treatment of the glenoid, this is referred to as a hemiarthroplasty. In shoulder resurfacing, the head of the humerus is ground smooth and covered with a metal cap (a resurfacing hemiarthroplasty), or just ground down. The glenoid fossa may also be ground smooth and covered with a plastic surface.
To accomplish shoulder replacement or resurfacing, a surgeon must gain surgical access to the joint. This is currently achieved from the front of the patient, in a deltopectoral approach, by cutting through the skin (a delta pectoral incision), separating the deltoid and pectoralis major muscles and pushing the cephalic vein aside, cutting through the subscapularis muscle on the front of the joint, cutting through conjoint tendon and the biceps tendon, cutting through circumflex blood vessels (arteries and veins), and cutting through the subscapularis tendon (or subscapularis muscle) and other tissues surrounding the joint to open to the joint capsule surrounding the humeral head. The joint capsule is then cut open to expose the humeral head and glenoid fossa.
With the humeral head exposed, the surgeon dislocates the shoulder to twist the humerus so that the humeral head points forward and is exposed, and pops it out of the shoulder to expose it to the reamers and saws needed to prepare the humeral head.
With the humeral head exposed, the surgeon will punch a hole in the intramedullary canal of the humerus to support a saw guide, and cut the round head of the humerus off using the saw guide to ensure a clean, flat cut across the anatomical neck. With the semispherical portion of the head removed, the surgeon inserts a long metal stem into the bone (into the intramedullary canal), and screws the metal half-ball onto the stem. This structure replaces the humeral head.
To replace the glenoid socket, the surgeon uses a reamer with a convex outer surface, to grind the glenoid socket to the shape desired to accommodate the glenoid socket prosthesis. The surgeon then drills several holes in the remaining glenoid socket to accommodate the several posts on the underside of the prosthesis. The surgeon inserts a drill guide into the exposed joint space, and then drill several holes in the glenoid socket. With the holes prepared, the surgeon inserts the glenoid socket prosthesis into the prepared glenoid socket. Access to the glenoid socket requires significant dislocation of the shoulder joint, to move the humerus out of the way.
In a resurfacing surgery, the head of the humerus is not cut off, but is instead ground smooth with a hemispherical reamer and covered with a metal cap. To accomplish this, the surgeon inserts a rigid guidewire into the center of the head of the exposed humerus, and drives the pin into the head, then through the humeral head. The surgeon then positions the hemispherical reamer over the humeral head and rotates it with a drill to grind the humeral head surface. The drill shaft extends outwardly from the apex of the humeral head, so that the reamer and drill can only be applied after dislocation of the shoulder to point the apex out of the joint. The inner surface of the reamer is the same shape as the cap, so that when the reaming is complete the remaining bone of the humeral head is the same shape as the inside of the cap. When reaming is complete, the surgeon secures the cap over the humeral head. The glenoid fossa may be replaced, resurfaced, or debrided to complete the procedure. Shoulder resurfacing is more conservative than should replacement because it requires removal of less bone from the humerus.
The procedure is usually successful in relieving pain and restoring some range of motion vis-à-vis the limited range of motion in the diseased shoulder. However, because muscles and tendons of the shoulder joint are cut, patients often suffer from severely restricted range of motion vis-à-vis the natural range of motion. Although any surgical exposure runs the low risk of injury to the neurovascular structures during surgical dissection and exposure, the anterior deltopectoral approach has a high incidence of subscapularis dysfunction (i.e. weakness) because of the transection of the tendon during the planned surgical exposure. By some reports, there is a 75% incidence of significant weakness of the subscapularis muscle even after a well-performed shoulder replacement.